Pressure sensitive adhesives (often referred to as PSA's) and methods for polymerizing compositions to produce pressure sensitive adhesives are known in the art. U.S. Pat. No. 4,181,752 (Martens et. al.), for example, discloses a one step process for preparing a normally tacky pressure sensitive adhesive by subjecting a radiation-sensitive acrylate containing polymerizable mass to radiation in the near UV region, at a fluence rate of from about 0.1 to 7 mW/cm2.
U.S. Pat. No. 6,174,931 B1 (Moon et. al.) discloses a multi-stage irradiation process for producing acrylic-based compositions such as adhesives or pressure sensitive adhesive tapes. The first irradiation stage employs electromagnetic radiation at a relatively low fluence rate (i.e. 0.1 to 20 mW/cm2) to effect a relatively low extent of monomer conversion, and the subsequent stage employs electromagnetic radiation at a relatively high fluence rate (i.e. greater than 20 mW/cm2) to substantially complete the photopolymerization.
U.S. Pat. No. 6,040,352 discloses a method of preparing an adhesive composition that includes exposing a free-radically polymerizable composition to a monochromatic radiation source having a peak spectral output at a wavelength falling within the range of about 250 nm to about 600 nm.
Low fluence rate ultraviolet radiation, for example from about 0.1 to about 20 mW/cm2, can be used to form high molecular weight acrylate polymers which are particularly useful as pressure sensitive adhesives having high performance characteristics such as, for example, high static shear strength. However, even after being exposed to radiation for long periods of time, there is typically a small amount of residual unpolymerized material left in the polymeric material that may be undesirable for some applications. A need therefore exists for improved methods for polymerizing compositions to produce pressure sensitive adhesives.
The present invention provides a method achieving a higher degree of polymerization, i.e. higher conversion, in significantly less time without sacrificing physical properties than can be achieved using previously known techniques using a single light source or combination of light sources. Surprisingly, it has been found that a second light source having most of its emissions in a wavelength ranging from between about 200 and about 280 nm is preferred in the practice of the invention even though evidence shows that such a light source by itself does not produce PSA's with high performance properties.